Analysis and Countermeasures of wire breakage of t

Analysis and Countermeasures of wire breakage of high-speed wire cutting machine (Part 2)

1.3 wire breakage related to pulse power supply

(1) large machining current and abnormal spark discharge lead to wire breakage. This kind of fault is mostly caused by the output of pulse power supply has changed to DC output. Check the waveform step by step from the output stage of the pulse power supply to the multivibrator, replace the damaged components, and put it into use when the output is the required pulse waveform

(2) the output current exceeds the limit and the wire is broken. During machining, the spark discharge suddenly turns into blue arc discharge, and the current exceeds the limit, burning the molybdenum wire. There is no waveform output at the input end and oscillation part measured by oscilloscope. It can be judged that the fault is in the oscillation part. It is found that there is a triode. There is an internal open circuit between the poles and an internal breakdown between the middle poles. Replace this tube and the high-frequency power supply will return to normal

another case is that the wire is suddenly broken during processing, and the current is above the limit. When measuring the output end of the high-frequency power supply with an oscilloscope, the waveform amplitude decreases and there is a negative wave, and the pulse width meets the requirements. The frequency, pulse width and amplitude of the measured driving stage waveform meet the requirements. Judge that the fault is in the power amplifier. Check the power tube and measure the internal breakdown between the CE poles of one of the tubes, so that the final current is directly added between the steel wire and the workpiece, causing the arc to burn the molybdenum wire. Replace the tube and return to normal

(3) burn points on molybdenum wires and wire breaks. Once there is a pimple like burn point on the molybdenum wire, it is very easy to break the wire. It is generally believed that this is caused by the processing chips (anode material) adhered to the electrode wire, which plays a role in concentrating the discharge on the electrode wire. At this time, if the cooling and heat dissipation conditions are poor, it is likely to raise the temperature there. In this way, in the continuous but unknown discharge, other processing chips may continue to adhere near the point, resulting in a vicious cycle, and finally lead to burns

as for why the machining chips adhere to the electrode wire, the main reason is related to the pulse parameters and the cooling condition of the discharge gap. The solution is to increase the no-load voltage amplitude of the pulse power supply, or adopt the double pulse method (similar to the commonly known grouping pulse), which can reduce the possibility of machining chips adhering to the electrode wire; Increase the coolant flow and improve the cooling conditions

(4) there is an ablation point on the molybdenum wire and the wire is broken. In the steel wire forehead, there is an ablation point every other section (about 10mm). It is slightly like a mildew, and the ablation point of molybdenum wire can be clearly seen in serious cases. This is caused by arcing between the electrode wire and the workpiece. For some reason, point a on the workpiece and point B on the molybdenum wire are arcing. The electrode wire is moving, and the arcing between points a and B is longer and longer, and point a and the closest point B start arcing. This cycle will form regularly spaced corrosion points, which will greatly reduce the strength of the electrode wire. The main reason for this phenomenon is that the output of the last stage of the feeding system is unbalanced. This phenomenon can be eliminated by adjusting the 1:7 taper hole feeding system at the bottom

1.4 wire breaks related to the wire feeding device and working fluid

(1) the root cause of wire breaks related to the wire feeding device is the poor accuracy of the device, especially the wear of different wheels, which will increase the jitter of molybdenum wire, destroy the normal gap of spark discharge, and easily cause large current centralized discharge, thus increasing the chance of wire breaks. The accuracy of the guide wheel mechanism can be checked from three aspects:

① the V-shaped groove of the guide wheel becomes wider. This will cause the electrode wire to move back and forth in the y-axis direction, which is manifested in the phenomenon of no feeding or jumping in when the wire storage cylinder is reversed

② the bottom diameter of the guide wheel V-groove is not round. This is because the bearing supporting the guide wheel is damaged, the molybdenum wire does not enter the V-shaped groove of the guide wheel or there is dirt to block the guide wheel during processing, and the molybdenum wire is pulled out of the deep groove. When shaking the wire storage cylinder by hand, it will be found that the electrode wire moves back and forth in the x-axis direction

③ poor contact between the conductive shaft and the conductive wheel causes wire breakage. During processing, it was found that the pointer of the ammeter swung from side to side, and the feed speed was uneven. Sometimes the pointer of the ammeter returned to zero, and the feed speed of the console was very fast. Because there was no discharge, the molybdenum wire was finally broken. At this time, replace the new conductive wheel and conductive shaft

(2) for workpieces requiring high cutting speed or large thickness, the proportion of working fluid can be appropriately diluted to a concentration of about 5% - 8%, so that the processing is more stable and it is not easy to break wires

(3) dirty working fluid and poor comprehensive performance after a long time are important reasons for wire breakage. In practice, we can measure whether the working fluid becomes worse in this way: when the processing current is about 2a, its cutting speed is about 40mm2/min, and it works for 8 hours every day, the effect is the best after two days of use. If it continues to be used for 8 ~ 10 days, it is easy to break the wire, and it is necessary to replace the working fluid

(4) some studies believe that the working fluid prepared with high-purity water works stably and has less broken wires. The reason is that it is estimated that some ions harmful to electrochemical processing, such as calcium ions and magnesium ions, have been removed from the high-purity water in the process of ion exchange purification, resulting in the stable processing due to the elimination of harmful ions, although there is the introduction of electrochemical products in the processing process, which makes the working fluid mixed with various ions

1.5 broken wires related to materials

(1) it is generally believed that metals without forging, quenching and tempering are easy to break wires. This is due to the uneven distribution of carbides in steel, which leads to the instability of electrical machining performance, resulting in arc discharge and wire breakage. The carbide particles contained in steel are large, agglomerated and unevenly distributed. Such materials are easy to crack and deform during processing, clamping the steel wire and causing wire breakage. Especially for quenched parts, there are about 500 ~ 800mp in carbon free alloy after quenching. In high carbon steel, the internal stress can reach 1600mpa. If it is ground, it can also cause 70 ~ 80mp. And EDM will form a white EDM metamorphic layer on its machined surface and produce a tensile stress of about 800MPa. The stress concentration caused by the interactive superposition of quenching stress, grinding stress and EDM stress is the direct cause of cracking and wire breakage in the cutting process of quenched parts. Therefore, in order to reduce wire breakage caused by materials, we should choose good forging performance and hardenability. Heat treatment of materials with small deformation promotes the uniform distribution of carbides in steel, thus enhancing the processing stability. Such as the cold stamping die with wire cutting as the main process; Try to select CrWMn, cr12mo, GCr15 and other alloy tool steels, and correctly select the hot working method and strictly implement the heat treatment specification

1.6 others

(1) during wire removal, the old wires on the wire storage drum are often cut off with scissors to form many short ends. If you don't pay attention to cleaning, the casting roller and the 3-roll calender will be mixed into the electrical parts or clamped in the wire feeding device, causing a short circuit and wire breakage

(2) after processing, turn off the processing power first, then turn off the working fluid, and let the wire run for a period of time before stopping. If the working fluid is turned off first, it will cause discharge in the air and form wire burning; If the wire feeding is turned off first, the wire speed is too slow or even stops running, resulting in poor wire cooling and lack of working fluid in the interval, which will also cause wire burning

(3) when manual cutting is required at some time (referring to the hand-operated cross sliding plate), the ammeter should be observed and the frequency conversion speed during normal cutting should not be exceeded, otherwise it is very easy to break the wire

(4) in practice, there is wire breakage caused by dirt, which often occurs after processing finished parts. Troubleshooting: tighten the real fixing screw behind the oil pump with a wrench. During the inspection, use the multimeter resistance block to measure the resistance between the steel wire and the machine tool shell of about 30K ohms, and the replacement pen has the phenomenon of charge and discharge. Further inspection found that there were many liquid slag dirt between the molybdenum wire stopper screw and the wire frame, and between the broken wire detection block and the screw on the fixed frame

after removing the stop and detection block, the charge and discharge phenomenon disappears and the resistance value increases. This is because the liquid slag dirt forms a resistance capacitance between the steel wire and the machine tool shell, which is connected in parallel on the output end of the power amplifier tube (because the shell and the power amplifier tube are equipotential), which widens the bottom of the output waveform and increases the DC component. When the workpiece is almost finished, the gap between the molybdenum wire and the workpiece increases the sampling potential, and the feeding speed is accelerated, which makes the molybdenum wire easy to break when the workpiece is finished. After cleaning the dirt, the machine tool returns to normal